Selective catalytic reduction (SCR) systems are increasingly used to reduce nitrogen oxides present in exhaust gas from internal combustion engines and particularly diesel engines. SCR systems store an SCR reactant in a liquid or solid state. The SCR reactant typically includes a combination of urea —(NH2)2CO— and water. An exemplary SCR reactant is ADBLUE, which is the registered trademark held by the German Association of Automobile Industry for an aqueous urea solution. The SCR reactant is delivered into a flow of exhaust gas downstream of an engine and upstream of one or more catalytic converters. A typical SCR system includes a selective discharge catalyst in an exhaust system, an injector to provide doses of the SCR reactant to the downstream catalyst, and an SCR reactant delivery system.
The SCR reactant delivery system includes a tank defining a main volume for the SCR reactant, and a reservoir structure disposed in the main volume of the tank and defining a cold start volume of the SCR reactant. Because the SCR reactant will freeze in some conditions, a volume of the frozen SCR reactant inside the reservoir structure is heated so as to melt the frozen SCR reactant to provide SCR for operating the engine in cold weather conditions. Current United States' Federal Regulations require the SCR reactant to be supplied to the exhaust flow within twenty minutes of cold engine startup in cold weather conditions of at least −40 degrees centigrade.